This invention relates generally to coating systems for protecting metals. More specifically, it is directed to slurry coating compositions for providing aluminum enrichment to the surface region of a metal substrate.
Many types of metals are used in industrial applications. When the application involves demanding operating conditions, specialty metals and alloys are often required. As an example, components within gas turbine engines operate in a high-temperature environment. The specialty alloys must withstand in-service temperatures in the range of about 650° C.-1200° C. Moreover, the alloys may be subjected to repeated temperature cycling, e.g., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating.
In the case of turbine engines, the substrate is often formed from a nickel-base or cobalt-base superalloy. The term “superalloy” is usually intended to embrace complex cobalt- or nickel-based alloys which include one or more other elements such as aluminum, tungsten, molybdenum, titanium, and iron. The quantity of each element in the alloy is carefully controlled to impart specific characteristics, e.g., environmental resistance and mechanical properties such as high-temperature strength. Aluminum is a particularly important component for many superalloys. It imparts environmental resistance to the alloys, and can also improve their precipitation-strengthening.
Superalloy substrates are often coated with protective metallic coatings. One example of the metallic coating is an MCrAI(X)-type material, where M is nickel, cobalt, or iron; and X is an element selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Another type of protective metallic coating is an aluminide material, such as nickel-aluminide or platinum-nickel-aluminide.
If the superalloy is exposed to an oxidizing atmosphere for an extended period of time, it can become depleted in aluminum. This is especially true when the particular superalloy component is used at the elevated temperatures described above. The aluminum loss can occur by way of various mechanisms. For example, aluminum can diffuse into the overlying protective coating; be consumed during oxidation of the protective coating; or be consumed during oxidation at the coating/substrate interface.
Since loss of aluminum can be detrimental to the integrity of the superalloy, techniques for countering such a loss have been investigated. At elevated temperatures, the substrate can be partially replenished with aluminum which diffuses from an adjacent MCrAlX coating. However, the amount of aluminum diffusion into the substrate from the MCrAlX coating may be insufficient.
One method for increasing the aluminum content of the superalloy substrate (i.e., in its surface region) is sometimes referred to in the art as “aluminiding” or “aluminizing”. In such a process, aluminum is introduced into the substrate by a variety of techniques. In the “pack aluminiding” process, the substrate is immersed within a mixture (or pack) containing the coating element source, filler material, and a halide activating agent. At high temperatures (usually about 700-750° C.), reactions within the mixture yield an aluminum-rich vapor which condenses onto the substrate surface. During a subsequent heat treatment, the condensed aluminum-based material diffuses into the substrate.
Slurry compositions are employed in another method for incorporating aluminum into the surface of a superalloy. For example, an aqueous or organic slurry containing aluminum in some form can be sprayed or otherwise coated onto the substrate. The volatile components are then evaporated, and the aluminum-containing component can be heated in a manner which causes the aluminum to diffuse into the substrate surface.
Important advantages are associated with using slurries for aluminizing the substrates. For example, slurries can be easily and economically prepared, and their aluminum content can be readily adjusted to meet the requirements for a particular substrate. Moreover, the slurries can be applied to the substrate by a number of different techniques, and their wetting ability helps to ensure relatively uniform aluminization.
Slurry compositions which contain aluminum are described, for example, in U.S. Pat. No. 3,248,251 (Allen). The aluminum particulates in the patent are dispersed in an aqueous, acidic bonding solution which also contains metal chromate, dichromate or molybdate, and phosphate. (The phosphate serves as a binder). The chromate ions are known to improve corrosion resistance. One prevalent theory described in U.S. Pat. No. 6,074,464 is that the chromate ions passivate the bonding solution toward aluminum, and inhibit the oxidation of metallic aluminum. This allows particulate aluminum to be combined with the bonding solution, without the undesirable reaction between the solution and the aluminum. The coatings described in the Allen patent are known to very effectively protect some types of metal substrates from oxidation and corrosion, particularly at high temperatures.
While the “Allen” compositions are useful for some applications, they have some disadvantages as well. One serious deficiency is that the compositions rely on the presence of chromates, which are considered toxic. In particular, hexavalent chromium is also considered to be a carcinogen. When compositions containing this form of chromium are used (e.g., in spray booths), special handling procedures have to be very closely followed, in order to satisfy health and safety regulations. The special handling procedures can often result in increased costs and decreased productivity.
Attempts have been made to formulate slurry compositions which do not rely on the presence of chromates. For example, U.S. Pat. No. 6,150,033 describes chromate-free coating compositions which are used to protect metal substrates such as stainless steel. Many of the compositions are based on an aqueous phosphoric acid bonding solution, which comprises a source of magnesium, zinc, and borate ions. The coatings are said to be very satisfactory, in terms of oxidation- and corrosion resistance.
However, the chromate-free slurry compositions may be accompanied by other serious drawbacks. For example, they are sometimes unstable over the course of several hours (or even several minutes), and may also generate unsuitable levels of gasses such as hydrogen. Furthermore, the compositions have been known to thicken or partially solidify during those time periods, making them very difficult to apply to a substrate, e.g., by spray techniques.
Moreover, the use of phosphoric acid in the compositions may also contribute to their instability. This is especially true when chromate compounds are not present, since the latter apparently passivate the surface of the aluminum particles. In the absence of the chromates, any phosphoric acid present may attack the aluminum metal in the slurry composition, rendering it thermally and physically unstable. At best, such a slurry composition will be difficult to store and apply to a substrate.
It is thus apparent that new slurry compositions useful for aluminizing metal substrates would be welcome in the art. The compositions should be capable of incorporating as much aluminum as necessary into the substrate. They should also be substantially free of chromate compounds—especially hexavalent chromium. (In some preferred embodiments, the compositions should also contain relatively low levels of phosphoric acid, e.g., less than about 10% by weight).
Moreover, these improved slurry compositions should be chemically and physically stable for extended periods of use and storage, as compared to the prior art. They should also be amenable to slurry-application by various techniques, such as spraying, painting, and the like. Furthermore, the use of these compositions should be generally compatible with other techniques which might be used to treat a particular metal substrate, e.g., a superalloy component.